The present invention relates generally to inertial sensors and, in particular, to a multiple output inertial sensing devices, which can ideally be used for motor vehicle safety systems.
Lateral inertial sensors, or accelerometers, are well known devices that sense acceleration generated from an external source such as a body to which they are attached. Accelerometers typically contain three main components. A first component is a mass, known in the art as a seismic mass or proof mass, that moves in response to the external body's acceleration. The proof mass is held in a rest position by a second component, which is a spring or a member that functions as a spring. A displacement transducer that measures the motion of the proof mass in response to acceleration is the third component. Upon acceleration, the mass moves from its rest position, either compressing or stretching the spring. The transducer detects the movement of the mass and converts the movement into an electrical output signal. The output signal, which may be amplified and filtered by signal conditioning electronics for more accurate measurement, is then transmitted to a control circuit or a control device that responds to the detected acceleration. The spring usually restricts the proof mass to movement in a single direction or axis. Accordingly, the accelerometer provides a directional acceleration signal. The three components of the accelerometer, the proof mass, the spring, and the transducer, are collectively known as a sensor or sense element.
Accelerometers typically utilize either a piezoelectric displacement transducer or a capacitive transducer. In piezoelectric displacement transducers, the motion of the proof mass is converted into the electrical output signal by the change in resistance of a piezoresistive material as it is expands or contracts. Piezoelectric transducers, however, have the disadvantage of being sensitive to heat and stress, which generally requires the use of expensive compensating electronic circuits. In capacitive transducers, the motion of the proof mass is converted by having the motion alter the capacitance of a member, which is then measured. Though capacitive transducers also have limitations, such as parasitic capacitance of their associated conditioning electronics, they are preferred because they are relatively unaffected by temperature and may be readily measured electronically.
Accelerometers have been used in many different applications, including vibration measurement, for example of rotating equipment, as well as shock measurement, inertial navigation systems and motor vehicle control systems. Conventional accelerometers, such as those disclosed in U.S. Pat. No. 4,945,765, are physically large and relatively expensive to produce. As a result, few conventional accelerometers have been installed on motor vehicles because of their size, weight, and cost.
Recently, semiconductor accelerometers have been developed that include the sense elements described above that are reduced in size and are mounted on a silicon chip. As a result semiconductor accelerometers are much smaller than conventional accelerometers and thus design options regarding the location of the accelerometer are more flexible. In addition, semiconductor accelerometers are less expensive to produce than the conventional accelerometers noted above.
Semiconductor accelerometers are typically manufactured utilizing either a bulk manufacturing technique or a surface manufacturing technique, both of which are well known in the art. Both bulk and surface manufacturing techniques are classified as Micromachined Electro Mechanical Systems (MEMS.) In bulk manufacturing techniques, the transducer and associated electronics are typically located external to the silicon chip. In surface manufacturing techniques, the transducer and electronics can be mounted on the silicon chip, further reducing the accelerometer's size requirements.
The use of accelerometers in motor vehicles, therefore, is becoming more prevalent. They have been used in modern automobile safety systems to sense changes in acceleration and provide a control signal for vehicle control systems, including speed control systems and antilock braking systems. They have also been used to sense crash conditions in order to provide a signal to trigger the release of vehicle supplemental restraint systems, more commonly known as airbags. Accelerometers in vehicle safety systems are typically calibrated to measure the changes in acceleration in terms of G-forces. One G-force, or “G”, is equal to the acceleration of gravity (9.8 m/s2 or 32.2 ft/s2.) Accelerometers utilized in vehicle control systems typically provide signals for “low” sensed accelerations in the range of 1.5G while accelerometers utilized in crash sensing systems typically provide signals for “high” sensed accelerations in the range of 40G.
In addition to sensing changes in lateral acceleration, motor vehicle control systems also utilize angular rate sensors to sense changes in angular velocity. Although angular rate sensors typically measure the change in rotational velocity of a vibrating ring rather than the change in acceleration of a proof mass, angular rate sensors also include transducers and are mounted upon silicon chips. Similar to accelerometers, angular rate sensors are utilized to supply control signals to vehicle control systems, such as, roll control systems and rollover sensing systems. Again, as with the lateral accelerometer described above, the vehicle control systems that utilize angular rate sensors can require different trigger levels. Thus, a roll control system responds to signals have smaller magnitudes than those utilized by a roll over sensing system. Angular rate sensors and accelerometers are known collectively hereinafter as inertial sensors.
Prior art motor vehicle safety systems that utilized semiconductor inertial sensors typically utilized a separate semiconductor inertial sensor for each control system. Each inertial sensor in turn required a separate Application Specific Integrated Circuit (ASIC) to process the sensor signal. Separate inertial sensors increased the overall cost of each system because each ASIC and inertial sensor had to be purchased individually. Each inertial sensor and ASIC required a fixed amount of space for installation, which correspondingly increased the size and reduced the design flexibility of the various control or safety systems due to the bulkiness of the inertial sensors.
As inertial sensor manufacturing technology becomes more mature, advances in MEMS technology continue to be realized. These ongoing technology developments in MEMS allow for the continued miniaturization and integration of the sense elements and electronics of semiconductor inertial sensors. It would be desirable, therefore, to utilize MEMS technology developments to reduce the size of the inertial sensors of the prior art. It also would be is desirable to reduce the number of inertial sensors and ASICs, to increase the design flexibility in locating the inertial sensors and ASICs, to reduce the overall size of the system, and to create inertial sensors that can be used as common components in various vehicle control systems.